The technology of cold forming is widely used in various sections of industry. There is a plurality of known methods (e.g. technological and tensile tests) for the evaluation of the cold-forming properties. In the course of technological tests, the suitability for a particular forming operation can be ascertained.
The recognition that there is a good interrelation between the quotient of the change of dimensions in longitudinal and lateral direction measurable by tensile test was an important step forward. This is the so-called anisotropy in normal direction, i.e. the r value and the result of the deep draw test (cupping) (Lankford, W. T & Associates: Trans.ASM 1950.42, page 1197 and Whiteley R. L. & Associates, Sheet Metal Industries 1961.5 pages 349-353).
The r value can also be used for the determination of the direction of ear formation or earing. The r value characterises the anisotropy of planar form changes of sheet metals because of the relationship: ##EQU1## If the value of .DELTA.r is positive, the direction of ear formation or earing is inclined to the direction of rolling of the sheet by an angle of 90.degree.; if it is negative, then this angle is 45.degree.. (The subscripts of r in the formula of interrelation mean the angles between the longitudinal direction of test pieces used for the determination of the value of r, and the direction of rolling).
Since, up to the present, there has been no other known characteristic that shows such a good interrelationship of the deep-drawing property of sheet metals, this index number has particular importance in the sorting of sheet metals produced for deep-drawing purposes.
The known processes for measuring the r value can be generally divided into two main groups. In the processes belonging to one of these groups accurate lines are formed in longitudinal and lateral directions on the surface of the test specimens, e.g. by Vickers-pyramids or by photographic methods. Thereafter, the test specimen is stretched by a tensile testing machine to a defined extent and after accurately determining (generally by a highly accurate measuring microscope) the changed length of the measuring lines the r value is obtained by calculation or by nomograms (e.g. Keeler S. P. Machinery 1968.4, pages 94/103).
These processes are labor intensive and unsuitable for measurements carried out in series. Their only advantage is that very little instrumentation is required.
In the processes of the other main group of methods, the longitudinal or lateral form changes of the test piece are continuously registered by electronic tensilometers of high accuracy. The r value is determined either by a suitably programmed digital computer (e.g. Roell-Korthaus Electronic Computer, Sheet Metal Industries 5, pages 450/451), or by a recording co-ordinatograph (e.g. see Hungarian Pat. No. 163,808).
These latter measuring processes eliminate the main drawbacks of the method of the former group. However, their general use is restricted because they require expensive computers or co-ordinatographs and specially trained skilled labor. They are useful therefore primarily in research work, but not the requirements of the material testing in production and application of sheet metals.